Crystallization apparatus having pressure-liquid level control means

ABSTRACT

Crystals are derived from a solution of a crystallizable compound by a thermal circulation system comprising upper and lower chambers, one chamber being under vacuum, the other under conditions other than vacuum. Heat is provided to promote vaporization of the solution and circulation of the liquid. Fresh solution of the compound to be crystallized is selectively supplied to one or both chambers.

United States Patent Walther 1 Jan. 7, 1975 [54] CRYSTALLIZATION APPARATUS HAVING 1,976,936 10/1934 Harms 159/45 PRESSUREJJIQUID LEVEL CONTROL 2,042,661 6/1936 Jeremiassen 159/45 MEANS 2,819,154 1/1958 Frejacques..... 23/273 R 2,876,182 3/1959 Hopper 23/273 R [75] Inventor: Hans Walther, Ladenburg, Germany 3, 67,162 9/1969 Putnam 1 159/45 3,514,263 5/1970 Malek 23/273 R 1 Asslgneei Bencklser, Chenflsche 3,518,061 6/1970 Laurentz.... 23/273 R Fabrlk GmbH, 3,585,237 6/1971 Terrana 23/273 R Ludwigshafen/Rhein, Germany [22] Filfidz 15 1972 Primary Examiner-Norman Yudkoff Assistant Examiner-S. J. Emery PP 280,928 Attorney, Agent, or FirmWeiser, Stapler & Spivak 521 US. Cl 23/273 R, 23/2726 5, 159/44, 1571 ABSTRACT 159/45, 159/27 A Crystals are derived from a solution of a crystallizable [51] Int. Cl Bold 9/02 compound y a t a lati n syst m mprising [58} Field of Search 23/273 R, 272.6 S; 159/45, pp n lower h m n h m r ng n r 159/27 A 44 vacuum, the other under conditions other than vacuum. Heat is provided to promote vaporization of the [56] References Cited solution and circulation of the liquid. Fresh solution of UNITED STATES PATENTS the compound to be crystallized is selectively supplied 1,831,121 11/1931 Kermar..... to one or both Chambers 1,879,445 11/1932 Othmer 159/45 15 Claims, 1 Drawing Figure VACUUM 8 UPPER RAW MATERlAL.

\NLET -5 3 1 I 47 1 15 1 I I I I -4 1 1 1 12 14 LOWER RAW MATERlAL \NLET COMPRESSED AlR OUT LET PIIIEIIIEIII mm VACUUM 8 1O UPPER RAWMATERIAI.

-' i \NLET V 13 i I T -5 I I I I 5 k w I I 15 I I I -4 I I 1'2. 14 LOWER RAW MATERIAL INLET COMPRESSED ou LET CRYSTALLIZATION APPARATUS HAVING PRESSURE-LIQUID LEVEL CONTROL MEANS The invention relates to crystallization equipment and, in particular, to such equipment which is capable of producing, by means of a crystallization process operating either intermittently or continuously, crystals of such compounds which normally exhibit a tendency to cake (adhering together) and/or to adhere to the equipment walls.

Known forms of continuously operation evaporation crystallizers are so constructed to yield a product of uniform grain size. They possess therefore a chamber in which the large crystals can settle out. In the case of crystals which have a tendency to cake and/or adhere to the walls of the equipment, use of a settling chamber quickly leads to malfunctions. Such conventional equipment, eg the so-called Oslo Crystallizer, and others are therefore unsuitable for the crystallization of such compounds. Accordingly equipment has to be provided in which the crystals have no opportunity to settle out, and therefore no opportunity to cake or stick to each other or to adhere to the equipment walls, but which nevertheless permits the production of crystals of sufficiently large size. Naturally, in order to provide a suitable apparatus which overcomes these difficulties care must also be taken that no crystalline growth appears in those portions of the equipment which normally are not continuously in contact with the solution. Of special concern in this respect, is the upper chamber of the apparatus where crystal growth can occur rapidly due to splashing of the solution.

According to the publication entitled Krystallisation," by D. Matz, publisher Springer, 2nd Ed. (1969), page 230, it is important, in all evaporation crystallization equipment employing heat exchangers, that no boiling takes place in the heating tubes to prevent crystal growth and deposits from taking place there. By preventing or depressing the formation of vapor bubbles in the heating tubes, the overall heat transfer coefficient is however decreased. In view of the above teaching, it was surprising to find in accordance with the present invention, that the heating tubes of the heat exhanger did not become encrusted, despite the formation of vapor bubbles. As a consequence of the formation of vapor bubbles in the heat exchanger tubes very satisfactory overall heat transfer coefficients are achieved upon vaporiazation.

It is accordingly an object of this invention to overcome the above-noted shortcomings of the prior art.

It is another object to provide crystallization equipment in which encrustation of the heat exchanger tubes is minimized.

It is another object to provide crystallization equipment in which good overall heat transfer is achieved.

It is another object to provide crystallization equipment capable of providing in trouble-free operation adequately large crystals from compounds which have a tendency to cake and/or have a tendency to adhere to the equipment walls.

It is yet another object of the invention to provide an apparatus and a process which operate reliably under the prescribed conditions to produce crystals of organic or inorganic chemicals or compounds of a predetermined, substantially uniform size, in accordance with operative conditions which are comparatively less complex than those of the prior art.

Other objects of the invention will become apparent from the further description of the invention.

In its broadest concept, the process of the invention comprises circulating a solution of the solid to be crystallized out between a vacuum zone and a crystalli2ation zone maintained preferably at no less than atmospheric pressure, desirably above it. The liquid is supplied with enough heat so that it will be vaporized in part. For best results, the liquid is circulated through a heating zone, theprocess comprising the additional aspect of heating the liquid, preferably prior to feeding it to the vacuum zone to a temperature high enough to vaporize a part of the solvent of the solution then passing th e liquid to the vacuum zone. In accordance with the invention the level of the liquid in the vacuum is kept constant. This is accomplished by regulating the pressure differential between the vacuum and crystallization zones in relation to the density of the crystal suspension. Difficult to crystallize materials are obtained as well formed crystals in a high degree of purity.

The apparatus and process of the invention is useful to crystallize out of solution (aqueous or from other organic solvents like alcohols, ester, ethers, hydrocarbons halogenated or not for instance parafinic hydro carbons) any crystalline compound or material. The invention is especially valuable in obtaining the acids which are normally obtained from fermentations like gluconic, itacanic, citric, fumaric, especially polycarboxylic acids, and their soluble salts. The process and apparatus are also very useful in crystallization of other organic acids like oxalic acid, adipic, malic, sartaric and other polycarbonylic acids or various salts thereof. There can be crystallized also other compounds for instance of amonium like the chloride, nitrate, phosphate, perchlorate, of potassium like the permanganate, chlorate; copper sulfate, sodium glutamate, borax, aniline salts, melamine, urea and others.

In accordance with a preferred embodiment of the invention, the crystallization equipment comprises an upper chamber and a lower crystallization chamber connected to each other by two conduits, the respective lower end of which are immersed in the solution within the lower crystallization chamber. The minimum length of these conduits is such that the suspension of crystals can be withdrawn under barometric conditions. One of the conduits contains a heat exchanger preferably in its upper portion and is so dimensioned that a flow velocity is obtained which is sufficient to entrain the crystals upwardly in the conduit.

The lower crystallization chamber contains means for providing agitation of the solution like an agitator. Means are also provided to affect and control the liquid level in the upper chamber to a constant level. The lower crystallization chamber is dimensioned, preferably sufficiently large to contain all the liquid which is present in the crystallization equipment. The solution can be fed selectively either into the lower crystallization chamber, or be delivered to the side walls of the upper chamber by means of a distribution arrangement having one or more inlets.

For further details reference may be had to the following discussion in the light of the accompanying single FIGURE of drawings, which illustrates a preferred crystallization equipment in accordance with the invention.

In this FIGURE there is shown upper chamber 1, lower crystallization chamber 2, descending conduit 3 and ascending conduit 4 with heat exchanger 5, agitator 6, lower raw material inlet 7 and upper raw material inlet 8 for selectively feeding the crystallization equipment according to the invention, outlet 9 for withdrawal of the crystal suspension, connection 10 to a vacuum pump (not shown), compressed air connection 11, and pressure relief valve 12 for regulating the pres sure in the lower crystallization chamber 2 with the aid of pressure regulator 14 which is actuated by level regulator l3, and finally differential pressure meter 15.

The individual elements of the crystallization equipment according to the invention, as well as their operation, are further described below.

in heat exchanger 5, the heat needed for vaporation of the solvent is supplied. As a result, a part of the solvent evaporates in the upper region of heat exchanger 5 and the vapor bubbles so created cause the circulation of the solution through upper chamber 1, chamber 2, through conduits 3 and 4, in accordance with the principle of mammoth-pump.

Heat exchanger 5 is best positioned in the uppermost part of ascending conduit 4 to promote the formation of vapor bubbles in the heating conduits and to bring about as high as possible an overall heat transfer coefficient by the resulting turbulence of the streaming liquid. Concurrently in this manner, there is prevented an overheating of the crystal suspension since the supplied heat is converted essentially immediately to vaporizing heat. The resulting somewaht more limited circulation rate is adequately compensated by these advantages. While circulation of liquids by means of rising vapor bubbles is, in itself, not new, the application of this principle to evaporation crystallizers is new, in accordance with the invention. On the contrary, as described above, for fear of encrustation, vapor bubble formation in the heat exchanger has heretofore been considered as specifically counter-indicated and undesirable. ln deed, in a search of the literature it has not been possible to find a crystallizer which operates as described herein. it was therefore very surprising that a crystal suspension which normally inherently tends to cake onto the walls does not lead to encrustation of the heat exchanger, in accordance with the invention.

For maximum efficiency it has been found advantageous that conduit 4 be so dimensioned that the upward velocity of the suspension is somewhat greater than the sedimentation rate of the crystals. Hence, the internal cross-section of ascending conduit 4 can be between 0.1 and 1.0 times the internal cross-section of all the tubes of heat exchanger 5, being preferably between 0.3 and 0.7 times.

it is also very desirable that ascending conduit 4 extend deep enough into lower crystallization chamber 2, so that a suspension rises in this conduit and also to prevent a breakthrough of the gas phase from chamber 2 into chamber 1. Should the velocity of ascent become so low that the crystal suspension begins to precipitate, then one can intermittently introduce additional raw material, in portions or all at once, into lower crystallization chamber 2. In this manner, the suspension flowing downwardly within descending conduit 3 is not diluted or diluted relatively little and the circulation rate isincreased as a result of the greater density of the descending suspension in conduit 3. This measure may be desirable case, for example, during start-up of the equipment. Lower raw material inlet 7 can be connected directly to the lower part of ascending conduit 4, or else to lower crystallizationichamber 2. it is preferable to connect inlet 7 to the upper part of the lower crystallization chamber 2. Since lower inlet 7 is only used intermittently, if the connection is made to ascending conduit 4, crystal lumps might tend to collect in the blind pipe section. It is therefore preferable to connect inlet 7 to the upper part of the lower crystallization chamber 2. Generally, fresh input of solutions is made to upper chamber 1 at 8.

in upper chamber 1, if wall encrustations take place, particularly near the surface of the liquid, and especially when its level is maintained nearly constant over long periods of time, they can be prevented very effectively by rinsing the walls with input of raw material. The upper raw material inlet 8 is made therefore desirably of a perforated pipe. Since the wall encrustations are not distributed uniformly around the circumference of upper chamber 1, upper raw material inlet 8 is preferably sub-divided into individualperforated pipe segments, which can be supplied with raw material in proportion to and selectively as there is need to minimize the development of encrustations of crystals. Although the raw material supply consists of a nearly saturated solution, rinsing of the walls suffices to keep crystal growth within desired limits.

Agitator 6 or other agitation means provided in the lower crystallization chamber 2, maintains the crystals in suspension and thus prevents, by its agitating action, the deposition or caking of crystals onto the walls of lower crystallization chamber 2. in the crystallization equipment embodying the invention, the greatest proportion of the crystallization takes place in lower crystallization chamber 2, can be demonstrated by measurement of the heat of crystallization developed therein. Although crystallization takes place principally in chamber 2 and agitator 6 keeps the crystals in suspension, there is obtained surprisingly at outlet 9 large crystals with satisfactory grain size distribution. Above all, however, the crystals also exhibit the desired hardness.

Lower crystallization chamber 2 is, according to a preferred aspect of the invention, so dimensioned to be capable of receiving the entire liquid content on the crystallization apparatus. This makes it unnecessary to empty the equipment during brief stoppages and permits, when operation resumes, rapid re-establishment of the original flow and equilibrium throughout the entire system. It then becomes unnecessary to go through the slow process of establishing the desired operating parameters which is otherwise required, as, for example, the density of crystal content of the suspension, the crystal size, etc. when a conventional apparatus is refilled with a fresh solution. Moreover, this dimensioning of lower crystallization chamber 2 is also advantageous for batch operation of the crystallization equipment. At the end of one charge, or batch, it is possible to release the partial vacuum in upper chamber 1, thereby draining its liquid content all at once into lower crystallization chamber 2, and avoiding the possible formation of crystal deposits during emptying, especially in heat exhanger 5.

For optimum results it is best when, in accordance with the invention, the liquid level in upper chamber 1 is maintained at such levelthat circulation over heat exchanger 5 and conduits 3 and 4 functions most effectively. This is accomplished, in conformance with a particularly noteworthy embodiment of the invention,

by forcing a portion of the suspension present in lower crystallization chamber 2 upwardly into upper chamber 1, whenever the liquid level therein is too low; and conversely draining an appropriate quantity of liquid into the lower crystallization chamber 2 when the liquid level in chamber 1 is too high. This can be readily accomplished by increasing the pressure as by introducing compressed air into the gas space above the liquid in lower crystallization chamber 2, or by appropriate decrease in pressure in this gas space. To this end, a compressed air inlet 11 is provided in lower crystallization chamber 2, and a pressure relief valve 12, which are actuated in conventional manner by pressure regulator 14. Activation of pressure regulator 14 is controlled, in turn, by level regulator 13, so that the liquid level in the upper chamber 1 is preferably continuously maintained at a constant desired elevation. Obviously, in lieu of compressed air some other gas can be utilized, e.g. an inert gas like freon.

By measuring (by means of differential pressure meter 15), the pressure difference between the gas spaces in upper chamber 1 and lower crystallization chamber 2, knowing the liquid level difference between the two chambers, it is readily feasible to determine the density of the crystal suspension, and therewith the crystal content, or alternatively its degree of evaporation or concentration. This is a very efficient procedure to run the process of the invention, which can be adapted to batch or continuous operations.

The crystal content of the suspension can be determined by reference to its specific gravity, as follows. It is well known that in communicating conduits the parameters of liquid depth, pressure and specific gravity are related as follows:

wherein H, and H are the heights of the liquid levels in the respective conduits, P, and P are the gas pressures to which these liquids are respectively subjected, and y is the specific gravity of the liquid.

Equation (1) above can be rewritten as follows:

In the equipment embodying the invention, P, and H, represent pressure 'and liquid level height in upper chamber 1 respectively, and P and H the corresponding values in lower chamber 2.

Levels H and H, are monitored, and their difference is determined. Pressure difference P, P is monitored by differential pressure meter 15. From these values the value of y, the specific gravity, hence the crystal content of the liquid is then readily derived.

The lengths of conduits 3 and 4 are primarily dependent on the vacuum in chamber 1 and from the specific gravity of the liquid to be crystallized and from the resulting crystalline suspension. Their lengths are so selected that there is established a level in chamber 1, which is preferably maintained constant, by applying a pressure slightly above the atmospheric pressure prevailing in the gas space in chamber 2. There is, of course, quite some leeway in the range of pressures in chamber 2, in the vacuum in chamber 1 and the position of the liquid level in chamber 1; hence the heights of the coluums can be adjusted to a practical length as may be best under the circumstances. Maintaining the pressure in chamber 2 at slightly over atmospheric eliminates the need for a pump for the removal of the crystalline product, the walls of the chamber can be of simpler or lighter-weight construction than with conventional equipment where the chamber of crystallization is under vacuum.

The apparatus and process of the invention combines therefore a unique operative arrangement in a vaporization zone under vacuum (or connected to a vacuum zone) and a crystallization zone under a pressure other than vacuum generally at a pressure slightly above atmospheric. The operation is unexpectedly efficient and successful.

Various modifications of the invention are possible without departing from the basic inventive concept. Though not necessary, an auxiliary pump, for example, can be installed in the equipment ot assist in circulating the crystalline suspension through the system if thermal circulation should prove inadequate to move a large mass of crystals. Likewise, compressed air can be introduced into ascending conduit 4 to promote circulation, as', for example, when heat supply is low in consequence of operating at less than full capacity.

The previously described relationship between specefic gravity pressure differences and level or niveau differences in the chambers, can also serve as the basis for automatic control of specific gravity of the liquid. Reference is made to equation (2), above, from which it can be seen that the specific gravity y becomes a function of the pressure difference P P, when the level difference H, H is maintained constant. On the other hand, by holding P P, constant, 7 becomes a function of the level difference H, H These relationships can be utilized as described below where several operative variants are described.

For the case in which the equipment operates in batch fashion, (which is also equivalent to start up operations), liquid is introduced into the system until the volume of the liquid adequately covers the bottom of inlet tube 3 and 4 so that there is prevented a breakthrough of the gases of the gas space in chamber 2 when the liquid level is raised in chamber 1. Thus the volume in chamber 2 is adequate to compensate initially for the volume occupied during operation in conduits 3, 4, heater 5 and chamber 1 and still prevent the gas of chamber 2 from entering the inlet tubes.

There is then established the vacuum in chamber 1, whereby the liquid rises into chamber 1 through conduits 3 and 4', the niveau in 2 drops correspondingly. When the desired vacuum in chamber 1 is reached, but if the niveau in 1 is below that desired, compressed air is introduced through duct 11 into 2; this causes a higher pressure on the liquid and the niveau in l rises to the level desired, where it is maintained. Now heat can be applied to 5 and the operation started. As heat is supplied, and evaporation progresses, the specific gravity of the solution increases. Fresh solution is fed to the system but only in such amount so that the pressure differential increases till it reaches the desired final value P P When this is reached, more fresh so lution is supplied to maintain level H, constant, at constant pressure difference P P, until the desired level height in chamber 2 is reached. The values which are realized then represent the condition at the end of the charge; the crystallization suspension has the desired specific gravity. The products can be separated or the process operated on a continuous basis.

In special cases, it is also possible after the end vacuum value has been reached in zone 1, to establish a preselected pressure difference P P, (by controlling the air pressure through valve 11), as is required to attain the desired final value of specific gravity of the suspension. Feeding of fresh solution continues until the desired liquid level H, in upper chamber 1 is reached. As evaporation progresses, the specific gravity increases denoting increasing crystalline concentration and the level of the liquid in the vacuum chamber drops. When this is observed, (as by means of level control 13), more fresh solution is added through 7 until the level H in chamber 2 has so been raised that level H, in chamber 1 has again reached the desired level. These conditions are maintained until the level H has reached the desired height in chamber 2, and the desired value for H, has been reached, upon concurrently maintaining the desired pressure difference P2 P When the equipment operates continuously, a predetermined concentration denoted by a particular preselected fixed value of specific gravity, can be maintained as follows.

The capacity of this apparatus is determined and regulated at a constant level difference H, H by feeding a constant volume of fresh solution. Controlled by the second level regulator connected to chamber 2, enough material is withdrawn from outlet 9 so that level H and thereby the operating volume are kept constant. As the specific gravity increases, the pressure in gas space of chamber 2 is raised (through valve 11) so that the level H, of the liquid in chamber 1 be maintained at the preselected level; a higher pressure difference P, P, is thus established. This variation is detected by control device 15 and indicates the rise in specific gravity of the solution.

In responseto the rise in pressure difference P P,, (which is detected by pressure regulator 15) the heat supply is decreased, (while maintaining constant the liquid supply), and thereby the vaporization capacity, in relation to the rising pressure differential P P,. As a result the specific gravity decreases again. Conversely, the pressure differential P P, decreases in response to a decrease of specific gravity. As a result, the heat supply is raised (while maintaining the liquid supply constant), and thereby the vaporization capacity. The specific gravity rises again.

Control equipment which can carry out the above operations is readily available. Pressure differences P P, can be detected by means of the differential pressure measuring device 15, which regulates the heat supply to heat exchanger 5. The pressure differential in itself is dependent on the position of valves 11 and 12.

Liquid level H, is controlled by level regulator 13 which is maintained constant by the operation of this regulator on pressure regulator 14. Level H is also maintained constant and is regulated by a separate level regulator connected to chamber 2 which, as described above, actuates outlet 9.

Another operating procedure is as follows. A particular pressure difference P P, is maintained. Should the specific gravity of the liquid rise above the desired value, H, decreases, thereby reducing the level difference H, H In response to this drop, raw material is fed in at supply 7, thereby raising both H, and H However, by withdrawal at outlet 9, H can be maintained simultaneously at a fixed level, and H,, as well as the difference H, H thereby restored (increased) to their desired values.

Conversely, when the specific gravity declines, H, H increases. Since H, itself is kept level by the supply of additional raw material, H drops. In response to this drop, the withdrawal of material through outlet 9 is interrupted until the desired difference H, H is again (decreased) restored.

Other details of the various embodiments of the apparatus and process of the invention are follows. The fresh raw liquid which is fed to the upper and/or lower desired chamber is preferably pre-heated to the operating temperature of the apparatus. In the case of aqueous solutions these temperatures range from about l5 to about 120 C and especially 30 to 60 C, which then also correspond to the operating temperature.

If desired, higher temperatures may also be used, for instance for non-aqueous solutions, of the material to be crystallized. The particular desired temperature depends in .general on the concentration of the raw material fed to the selected chamber, the solubility of the solid to be crystallized out thereof, the boiling point of the solvent (which is a practical upper operating limit of the process) and other similar factors. The upper concentration of the raw material fed is determined by the saturation point of the solid to be crystallized out at the prevailing temperature. The concentration of the solid is usually adjusted to values best suited under the circumstances. Commonly, the concentration is from about 25 to about 90%; it being 25 to 70% for citric acid. Vacuum zone 1 is maintained at a pressure below atmospheric, which is in general determined by the desired crystallization temperature. The pressure is generally in the range of about 10 to 760 Torr, for aqueous solutions, specially at about 20 to 55 Torr.

It is a noteworthy aspect of the process of the invention and of the apparatus therefor that it is particularly well-suited for the crystallization of organic and inorganic chemicals which are known to require a long residence time for the formation of large crystals, for instance, citric acid, and other compounds disclosed above. It is another noteworthy aspect of the apparatus and process of the invention that it does not rely on any pump system or mechanical means for transferring the liquid or any of the materials from one level to another or from one chamber to the other. Circulation is provided by the vaporization and thereby the resulting difference of the average specific gravity in conduits 4 and heater 5, on one hand, and conduit 3 on the other created.

It is to be noted that in the embodiment of the invention illustrated, the vacuum and the crystallization zones are illustrated as being in two different horizontal planes. But this is not a requirement of the apparatus or process of the invention. The containers or cham bers can be positioned on the same or substantially same plane providing that there prevails an adequate level differential between the liquids in the respective chambers. The vertical separation between chambers l and 2 is relative to the vacuum in chamber 1, the gas pressure in chamber 2 and from the specific gravity of the crystallization suspension.

It is to be noted too that a heating zone as such need not actually be part of the apparatus or process of the invention as long as the liquid which is supplied to the system is at a temperature high enough to cause vaporization of the solvent when introduced into the vacuum zone. Thus instead of having heating zone 5, there may be provided a suitable inlet for liquid at high temperature, high enough that when fed into the vacuum zone it will vaporize the solvent as described above. Any suitable energy means to change the liquid to vapor is suitable. The crystallization product can be removed from the chamber continuously or intermittently.

The process of the invention is well-suited for fractional crystallization where a series of these crystalliza tion and vacuum chambers may be coupled in series and thereby achieving a sequential crystallization from liquids containing more than one particular solid. In such an arrangement the liquid from chamber 2 once it has been substantially freed from the first solid material, is then passed onto another chamber 2 which is then connected to a system similar to the one described above. Instead of connecting and passing the liquid from chamber 2 into a similarly operated system, the system of the present invention lends itself admirably to the separation of one solid as a crystal from the liquid, and then continuing the process for the separation of the second crystal.

In accordance with the process and the apparatus of the invention the crystallization chamber can be maintained at either atmospheric pressure or under a positive pressure. What is necessary is that there be established a differential in pressure between the gas space of the crystallization chamber and the vacuum chambers. It is preferable that the pressure in the crystallization chamber be at least atmospheric pressure.

The invention is illustrated by the following illustrative example, which is non-limiting.

The process is operated with a solution of citric acid, the operating parameters are as follows. The pressure and vacuum are, respectively: P 1.25 ata and P 55 Torr Hence P 1.25 ata 12.5 meter (water column height) P 55 Torr 0.75 meter (water column height) The crystallization suspension of citric acid has a specific gravity of y 1.4

From formula (2) above, there is derived A H 12.5 0.75/14 =1 l.75/l.4 8.4 meters Thus the process is operated with a level difference of 8.4 meters between the two liquids in the respective chambers.

When the process is operated with the crystallization chamber at atmospheric pressure, i.e.,

P =1 atm. A 760 mm Hg =0.33 m Ws AH 6.84 m

The versatility of the apparatus acid process is noteworthy in as much as it is operative at varying ranges of pressures, both super-atmospheric in the crystallization chamber and under atmospheric in chamber 1. It is generally desired to maintain the crystallization chamber at a pressure slightly above atmospheric, as in the range of about 1.0 to about 2.0 ata, dependant of the specific gravity of the solution and the difference between the levels in the two chambers. The difference in levels of the two liquids is also dependent not only on the differential pressures in the air spaces in the respective chambers, but also on the specific gravity of the liquid to be crystallized, during the process and at the saturation point the crystals. When the process is operated to crystallize other chemicals having respective values, the pressures and vacuum, and/or the level differentials are adjusted accordingly.

It is to be noted that instead of having agitation means 6 which is now represented as a stirrer, there could be provided other suitable means agitation or stirring means effective to keep the crystals in suspen sion, such as air bubbling.

Chamber 2 can be opened and operated to atmo spheric pressure and during all or part of the process; in that case the level in vacuum chamber 1 is given by the vacuum thereat and by the specific gravity of the suspension.

The process of the invention is operative with any liquid from which it is desired to crystallize a solid. It is specially useful in the crystallization of organic acids including hydroxy-substituted organic acids, traditionally obtained from fermentations, like itaconic, fumaric, gluconic, citric acids. and their soluble salts es pecially the alkali metal salts such as sodium, potassium, ammonium and the like. The crystalls obtained by the process of apparaties are wellformed crystals which, unlike the prior art products, remain in the original form when and as formed, with minimum breakage or physical attrition. Moreover, the crystals of the invention are well-developed, they are substantially free of smaller crystals and the greatest majority of the crystals are larger crystals rather than smaller crystals. These characterisitic of the products make the apparatus and process of the invention an important contribution to the arts.

I claim:

1. A crystallization apparatus for a liquid containing a material in solution comprising;

a vacuum and a crystallization chambers;

first and second conduit means connecting said chambers, and said liquid partially filling each of said chambers and circulating from said vacuum to said crystallization chamber through said first conduit means and from said crystallization to said vacuum chamber through said second conduit means;

means for establishing in said vacuum chamber a vacuum; and said crystallization chamber having means adapted to control the liquid level in said crystallization chamber relative to the liquid level in the vacuum chamber said last-named means being adapted with pressure means controlling the pressure in said crystallization chamber above atmospheric thereby controlling the liquid levels in the crystallization chamber and the vacuum chamber at predetermined levels with respect to each other; said levels being adjusted when required by changes in the density of the liquid being crystallized; and

means for causing in said second conduit means the flow of vapor bubbles of said solution toward said vacuum chamber.

2. The apparatus of claim 1 which is provided with agitation means in the crystallization chamber.

3. The apparatus of claim 1 which is provided with inlet and outlet means.

4. The apparatus of claim 1 wherein the crystallization chamber is provided with pressure relief means.

5. The apparatus of claim 1 wherein the crystallization chamber is provided with evacuating means for a suspension of crystals.

6. The apparatus of claim 1 in which the crystallization chamber is provided with inlet means for fresh liquid.

7. The apparatus of claim 1 wherein the crystallization chamber is dimensioned to be of such capacity as to be able to contain all of the volume of liquid from which the crystals are to be crystallized.

8. The apparatus of claim 1 which is provided with automatic means for controlling the liquid level in the crystallization chamber at a preselected level in dependence on the level of the liquid in the vacuum chamber.

9. The apparatus of claim 1, wherein said bubble causing means comprises heating means for vaporizing a portion of the solution flowing through said second conduit means.

10. The crystallization apparatus of claim 9 wherein the heating means is in closer proximity to the vacuum chamber than to the crystallization chamber.

11. The apparatus of claim 9 wherein the open end of the conduit means leading from the crystallization chamber to the heating means extends into the liquid in the crystallization chamber.

12. A crystallization apparatus for a liquid containing a material in solution to be crystallized comprising;

a vacuum and a crystallization chambers;

first and second conduit means connecting said chambers, said chambers being adapted to be partially filled by the liquid and said first conduit means being adapted for circulation for the liquid circulation of the liquid from said crystallization to said vacuum chamber;

means for establishing in said vacuum chamber a vacuum; and said crystallization chamber being adapted with pressure means controlling the pressure in said crystallization chamber above atmospheric thereby controlling the liquid levels in the crystallization chamber and the vacuum chamber at predetermined levels with respect to each other; said levels being adjusted when required by changes in the density of the liquid being crystallized; and

heating means for promoting in said second conduit means the flow of vapor bubbles of said solution toward said vacuum chamber.

13. The apparatus of claim 12 which is adapted to be operated at constant temperature by adjustment of the pressure means of the crystallization chamber and the pressure means of the vacuum chamber while the spe cific gravity of the liquid increases.

14. The apparatus of claiim 12 which is adapted to be operated to maintain a constant specific gravity by ad justment of the pressure means of the crystallization chamber and the pressure means of the vacuum chamber while different crystallization temperatures are established.

15. The apparatus of claim 12 wherein the heating means is positioned in the uppermost part ofthe second 

1. A CRYSTALIZATION APPARATUS FOR A LIQUID CONTAINGING A MATERIAL IN SOLUTION COMPRISING; A VACUUM AND A CRYSTALLIZATION CHAMBERS; FIRST AND SECOND CONDUIT MEANS CONNECTING SAID CHAMBERS, AND SAID LIQUID PARTIALLY FILLING EACH OF SAID CHAMBERS AND CIRCULATING FROM SAID VACUMM TO SAID CRYSTALIZATION CHAMBER THROUGH SAID FIRST CONDUIT MEANS AND FROM SAID CRYSTALIZATION TO SAID VACUMM CHAMBER THROUGH SAID SECOND CONDUIT MEANS; MEANS FOR ESTABLISHING IN SAID VACUUM CHAMBER A VACUUM; AND SAID CRYSTALIZATION CHAMBER HAVING MEANS ADAPTED TO CONTROL THE LIQUID LEVEL IN SAID CRYSTALLIZATION CHAMBER RELATIVE TO THE LIQUID LEVEL IN THE VACUUM CHAMBER SAID LAST-NAMED MEANS BEING ADAPTED WITH PRESSURE MEANS CONTROLLING THE PRESSURE IN SAID CRYSTALLIZATION CHAMBER ABOVE ATMOSPHERIC THEREBY CONTROLLING THE LIQUID LEVELS IN THE CRYSTALLIZATION CHAMBER AND THE VACUUM CHAMBER AT PREDETERMINED LEVELS WITH RESPECT TO EACH OTHER; SAID LEVELS BEING ADJUSTED WHEN REQUIRED BY CHANGES IN THE DENSITY OF THE LIQUID BEING CRYSTALLIZED; AND MEANS FOR CAUSING IN SAID SECOND CONDUIT MEANS THE FLOW OF VAPOUR BUBBLES OF SAID SOLUTION TOWARD SAID VACUUM CHAMBER.
 2. The apparatus of claim 1 which is provided with agitation means in the crystallization chamber.
 3. The apparatus of claim 1 which is provided with inlet and outlet means.
 4. The apparatus of claim 1 wherein the crystallization chamber is provided with pressure relief means.
 5. The apparatus of claim 1 wherein the crystallization chamber is provided with evacuating means for a suspension of crystals.
 6. The apparatus of claim 1 in which the crystallization chamber is provided with inlet means for fresh liquid.
 7. The apparatus of claim 1 wherein the crystallization chamber is dimensioned to be of such capacity as to be able to contain all of the volume of liquid from which the crystals are to be crystallized.
 8. The apparatus of claim 1 which is provided with automatic means for controlling the liquid level in the crystallization chamber at a preselected level in dependence on the level of the liquid in the vacuum chamber.
 9. The apparatus of claim 1, wherein said bubble causing means comprises heating means for vaporizing a portion of the solution flowing through said second conduit means.
 10. The crystallization apparatus of claim 9 wherein the heating means is in closer proximity to the vacuum chamber than to the crystallization chamber.
 11. The apparatus of claim 9 wherein the open end of the conduit means leading from the crystallization chamber to the heating means extends into the liquid in the crystallization chamber.
 12. A crystallization apparatus for a liquid containing a material in solution to be crystallized comprising; a vacuum and a crystallization chambers; first and second conduit means connecting said chambers, said chambers being adapted to be partially filled by the liquid and said first conduit means being adapted for circulation for the liquid from said vacuum to said crystallization chamber and said second conduit means being adapted for circulation of the liquid from said crystallization to said vacuum chamber; means for establishing in said vacuum chamber a vacuum; and said crystallization chamber being adapted with pressure means controlling the pressure in said crystallization chamber above atmospheric thereby controlling the liquid levels in the crystallization chamber and the vacuum chamber at predetermined levels with respect to each other; said levels being adjusted when required by changes in the density of the liquid being crystallized; and heating means for promoting in said second conduit means the flow of vapor bubbles of said solution toward said vacuum chamber.
 13. The apparatus of claim 12 which is adapted to be operated at constant temperature by adjustment of the pressure means of the crystallization chamber and the pressure means of the vacuum chamber while the specific gravity of the liquid increases.
 14. The apparatus of claiim 12 which is adapted to be operated to maintain a constant specific gravity by adjustment of the pressure means of the crystallization chamber and the pressure means of the vacuum chamber while different crystallization temperatures are established.
 15. The apparatus of claim 12 wherein the heating means is positioned in the uppermost part of the second conduit means. 